1. Field of the Invention
This invention is directed to a method and apparatus for processing fly ash from coal burning facilities. More particularly, the invention relates to thermally processing variable carbon content fly ash so that it becomes fly ash with a substantially constant carbon content useable as an additive for concrete.
2. Description of Prior Art
It has been known for some time that fly ash with a sufficiently low carbon content was extremely useful as an additive to concrete. The fly ash significantly strengthens the concrete, and extends the useful life of the concrete. However, the carbon content of the fly ash affects the air content and the water demand of the concrete mixture. High carbon content ash requires more air entraining agent to be used in the concrete mixture, and also requires more water for the concrete mixture.
Most concrete specifications require the Loss On Ignition (LOI) value for fly ash in concrete to be less than 5%. The LOI is a measure of the carbon content of the fly ash. For consistency in quality concrete, many state highway departments require an LOI of less than 3%.
The fly ash typically produced by coal burning facilities, such as power plants, is not controlled as to its carbon content, and may have an LOI as high as 10%. Such fly ash is not useable and must be disposed off in an environmentally safe manner. Because of the value of fly ash as a concrete additive and because of the need to dispose of vast quantities of fly ash produced each day, there have been a number of attempts at lowering the carbon content of fly ash. To date, none of these processes has produced a fly ash that is consistently low in carbon content.
One prior process uses microwave energy to heat the carbon in fly ash to combustion. An example of such a device is described in U.S. Pat. No. 4,549,053 issued to G. Haugh. The difficulty with the Haugh device is that it is not possible to uniformly control the temperature of the carbon particles. Some of the carbon is heated sufficiently to burn and other particles are not. It is better to provide a combustion chamber to heat all of the fly ash.
Heating fly ash in a combustion chamber has been tried by introducing flammable gases into the combustion chamber. The difficulty with this technique is that it is expensive and produces undesirable gas side products that must then be scrubbed from the fly ash and gas mixture after the burn.
A prototype device for electrically heating fly ash to the combustion point was created by the present inventor, and described in a publication in Aberdeen's Concrete Trader in August, 1991, entitled "Thermal Processor Burns OFF Carbon To Reduce Fly Ash Variability," written by B. A. Suprenant. A similar article was republished at pages 114-118 of the February, 1992, issue of Aberdeen's Concrete Construction. In this prototype device, the fly ash is dragged through an electrically heated pre-heat chamber and then through a combustion chamber. While the prototype device was superior to the gas fired chamber and to microwave devices for burning fly ash, it still did not achieve a consistent burn of the carbon in the fly ash.